Computer Numerical Control (CNC) machine tools are generally known in the art for use in machining various workpieces for a variety of uses. CNC tools commonly comprise a tooling center that secures the workpiece to be machined, as well as providing a plurality of tools for machining various internal and/or external features of the workpiece, as desired. The CNC tooling center machines the workpiece as directed by the computer, in response to computer aided design or computer aided manufacturing instructions.
One type of CNC machine tool is a CNC turning center, such as the Mazak QTN450MY 3000U machine tool, provided by the applicants' assignee herein, Mazak Corporation. A CNC turning center generally includes a headstock, for securing one end of a workpiece and rotationally driving the workpiece, a tailstock for securing the opposite end of the workpiece, and a turret for holding multiple cutting tools that can be selected during machining to perform various machining operations on the workpiece. Some turning centers include a steady-rest that can selectively support the workpiece between the headstock and the tailstock to provide additional support to an elongated body of the workpiece during machining.
In general, the workpiece is secured at each end and turned via the headstock, which can rotate the workpiece at a predetermined speed. The tailstock can rotate in accordance with the rotation of the workpiece caused by the headstock. Likewise, the steady rest can allow the workpiece to rotate while the steady rest supports the workpiece between the headstock and the tailstock.
The turret can travel along the axis of the workpiece to machine various axial locations along the outer surface of the workpiece between the headstock and the tailstock. The turret holds multiple cutting tools that can machine the outer surface of the workpiece. Additionally, the turret may hold a boring tool that can machine the interior of the workpiece when the turret is translated beyond the end of the workpiece supported by the tailstock. In a typical CNC turning center, the turret is a generally disc shaped member mounted for rotation on a carriage. These tools extend from the perimeter of the turret for selection via rotation of the turret. However, because the boring tool is elongated, the boring tool axis will extend perpendicularly to the plane of the disc-shaped member of the turret and generally parallel to the rotary axis of the turret and the central axis of the elongate workpiece. The boring tool mounted to the turret extends toward the driven end of the workpiece held by the headstock.
A turning center having a turret allows for multiple machining operations due to the number of different tools that are attached to the turret. Based on the programming of the computer, the turning center can switch from using one external cutting tool to another, and subsequently switch from an external cutting tool to the boring tool. This tool switching allows for complex machining of a workpiece without requiring an operator to change out tools, providing for an efficient process.
The boring tool is typically an elongated cylindrical bar having a cutting tip disposed at one end. The depth that the boring tool can machine inside the workpiece is limited by the length of the boring tool. The longer the boring tool, the deeper the boring tool can bore within the workpiece.
However, having the turret carry the boring tool limits the range of machining on the workpiece. The typical turning center is generally enclosed to keep debris and machining coolant from entering the air and to shield the machinist from such debris. This allows the machinist to closely monitor the machining process, but limits the amount of space within which the tools can operate. The headstock is located at one end of the machine and includes a rotary drive for rotating the workpiece. When the boring tool is mounted at the turret, it extends toward the headstock. Because the boring tool extends toward the headstock, the turret cannot travel closer to the headstock than the length of the boring tool. In the Mazak QTN450MY 3000U machine tool, the length of the boring tool is restricted to 210 mm to allow the full length of the workpiece, and particularly portions thereof near the headstock to be machined by the external cutting tools carried by the turret. In other words, if the boring tool was longer than 201 mm, then external machining operations on portions of the workpiece adjacent the headstock would not be able to be undertaken due to the interference between the longer boring tool and the headstock. For example, if the boring tool extends 500 mm from turret, there is an axial window within which exterior machining of the elongated workpiece cannot be performed. In this example, the window is about 290 mm (500 mm-210 mm). Therefore, mounting a longer boring tool to the turret limits the axial machining range of the turret along the elongated workpiece.
For the boring tool to machine the interior of the workpiece, the turret must travel beyond the end of the workpiece engaged by the tailstock. The turret must be able to clear the end of the workpiece at least by the distance that the boring tool extends from the turret. As discussed above, in the QTN450MY 3000U machine tool, the maximum length of the boring tool is about 210 mm without restricting the length of the workpiece that can have exterior cutting operations performed thereon. The maximum length of the workpiece is about 3331 mm, minus the chucking length at the headstock and the length of the boring tool. In one form, a 21 inch diameter chuck has a chucking length of 254 mm. Thus, a 210 mm boring tool would require a reduction in the length of the workpiece to 2867 mm. Increasing the length of the boring tool to 500 mm would reduce the length of the workpiece to 2577 mm. Therefore, mounting the boring tool to the turret requires a decreased length of the workpiece between centers if boring operations are desired. The longer the boring tool, the shorter the workpiece must be to allow for the boring tool to sufficiently clear the tailstock end of the workpiece. To perform deep boring operations without sacrificing the length of the workpiece, the workpiece would need to be removed from the turning center and machined using a different machine tool dedicated to deep boring.
Moreover, the turret has a limited size in which cutting tools or boring tools can be mounted. Mounting the boring tool to the turret typically results in a decrease of the number of cutting tools that can also be mounted to the turret. Increasing the diameter of the boring tool further limits the number of other tools that can be mounted to the turret. Decreasing the number of cutting tools reduces the number of machining operations that can be performed. In the QTN450MY 3000U machine tool, the diameter of the boring tool at the turret is about 50 mm.
Additionally, the boring tool length is limited by the manner in which it is secured. As mentioned, the turret is generally disc-shaped, and thus the area of the boring tool held by the turret is generally limited to the thickness of the disc-shaped turret. As the length of the boring tool is increased, the boring tool has a greater length thereof that is cantilevered from the relatively thin disc-shaped body of the turret so that there is an increase in the instability of the boring tool due to the limited clamping area the turret provides. In the QTN450MY 3000U machine tool, the maximum diameter to length ratio of the boring tool when mounted to the turret has been found to be about 1:4.2 before the boring tool would become unstable during interior machining, resulting in chatter. Increasing the diameter of the boring tool to 100 mm, would allow for increased length of the boring tool to 420 mm, but this increased length would restrict the range of machining and the length of the workpiece, as described above, in addition to further limiting the area at the turret to mount cutting tools.
Thus, the length of the boring tool or the length of the workpiece is restricted. First, with the boring tool extending orthogonally from the face of the turret, the length of the boring tool limits the distance that the turret can translate toward the headstock, thereby limiting the axial machining range of the cutting tools held at the turret. Second, the elongate boring tool is mounted to the disc-shaped body of the turret to be cantilevered therefrom, such that the portion of the boring tool that is held by the disc-shaped body is limited to the thickness of the disc-shaped body of the turret. The limited support of the boring tool limits the length of the boring tool to avoid instability of the boring tool during machining. Third, for the boring tool to machine the interior of the workpiece, the turret needs to translate beyond the end of the workpiece at the tailstock end a distance far enough to permit the entire length of the boring tool to be beyond the end of the workpiece. Thus, for a given size of the turning center, a longer boring tool will typically require a shorter workpiece.